Wilson, much like Hemingway himself, greatly values masculinity and a masculine identity, and he places great stock in the idea that one must ‘become a man’ by letting go of fear.
Wilson views American men as unmanly. He suggests that American men have “adolescent” faces, and “boyish” figures, and later in the story it is made explicit that Wilson sees this as a manifestation of their failure to become men: either for a long time, or forever (“sometimes [they remain boys] all their lives”).
It’s important to realize that Wilson is familiar with a very specific type of American man: one that is both wealthy enough to go on safari, and who has the motivation to do so. We could say then that Wilson has a biased sample on which to base his opinion: he has only really met well-to-do American men who have some motivation to partake in the hyper-masculine activity of hunting. In other words, it may be that this particular type of “adolescent” American man is seeking out hunting (and Wilson’s services) as a way to feel more manly.
The same could be said about Wilson’s opinion of American women. He describes them as “the hardest, the cruelest, the most predatory and the most attractive”, and speculates that this is why “their men have softened or gone to pieces nervously”. If it is a particular kind of American man that goes on safari, then this kind of man may be more likely to have a ‘cruel’ (as Wilson sees it) wife.
In short, Wilson’s opinions of American men and women are probably due to the fact that, as a safari guide, he is exposed to only a very small sample of the kinds of American men and women that exist in America as a whole. Having said this, it’s quite clear that Hemingway’s own views on masculinity and manliness emerge within Wilson: so much of Hemingway’s work focuses on men as they seek a sense of manliness or grapple with their unmanliness (ex. Jake Barnes in The Sun Also Rises), and I would suggest that Hemingway has his own disgust with what he sees as “unmanly” men.